But isn’t that already the case? Every library whose source I’ve studied has all kinds of detection, whether it’s feature or browser detection, in order to work with multiple browsers. I would think that under version targeting, the same thing would be necessary: you do feature detection and work accordingly. Again, it’s entirely possible I missed something there, so feel free to let me know what it was.
If there happen to be any differences in the way IE7 mode is implemented, the library may even need to cope with IE10’s IE7 mode and IE11’s IE7 mode. If the library does anything with CSS (for example a drag and drop script) variations in CSS rendering across all those implementations has to be taken into account too.
Developing for the current browser plus the previous version is inconvenient but readily achievable. Developing for the current version, the previous version and a version 4 years old is not only a nightmare, but also limits the functionality you can use and slows the progress of the web.
With version targeting, IE7 will never go away. Just as browsers are born, they must also die and make way for the next generation.